benchmark: ensure all watcher receivers to finish
benchmark: ensure all watcher receivers to finish

Fix https://github.com/coreos/etcd/issues/5099.

dannysauer/etcd,linuxcer/etcd,tguangch/etcd,akauppi/etcd,endocode/etcd,xiang90/etcd,rnaveiras/etcd,StorReduce/etcd,ravigadde/etcd,jlamillan/etcd,etcd-io/etcd,armstrongli/etcd,yudai/etcd,glevand/coreos--etcd,cockroachdb/etcd,sebrandon1/etcd,nekto0n/etcd,ktateish/etcd,joshix/etcd,YuleiXiao/etcd,visheshnp/etcd,tguangch/etcd,luxas/etcd,vimalk78/etcd,chipironcin/etcd,sofuture/etcd,purpleidea/etcd,li-ang/etcd,xiang90/etcd,chipironcin/etcd,CrazyJvm/etcd,insequent/etcd,swingbach/etcd,mkumatag/etcd,heyitsanthony/etcd,dannysauer/etcd,mickep76/etcd,hongchaodeng/etcd,rnaveiras/etcd,lorneli/etcd,whuwxl/etcd,sinsharat/etcd,mkumatag/etcd,wantonsolutions/dinv-etcd,achanda/etcd,swingbach/etcd,fanminshi/etcd,mqliang/etcd,fanminshi/etcd,visheshnp/etcd,lumjjb/etcd,wantonsolutions/dinv-etcd,wantonsolutions/dinv-etcd,rchicoli/etcd,mqliang/etcd,gyuho/etcd,glevand/coreos--etcd,a-robinson/etcd,lorneli/etcd,rchicoli/etcd,OyTao/etcd_learning,nekto0n/etcd,qqsun8819/etcd,mkumatag/etcd,bluebreezecf/etcd,plasticbox/etcd,a-robinson/etcd,qqsun8819/etcd,vimalk78/etcd,ktateish/etcd,jonboulle/etcd,StorReduce/etcd,dnaeon/etcd,gyuho/etcd,armstrongli/etcd,westhood/etcd,achanda/etcd,purpleidea/etcd,coreos/etcd,bdarnell/etcd,handsomesun/etcd,mqliang/etcd,endocode/etcd,etcd-io/etcd,ravigadde/etcd,li-ang/etcd,OyTao/etcd_learning,coreos/etcd,CrazyJvm/etcd,cockroachdb/etcd,heyitsanthony/etcd,dnaeon/etcd,handsomesun/etcd,glycerine/etcd,yudai/etcd,ajityagaty/etcd,zaphod-concur/etcd,joshix/etcd,lumjjb/etcd,zaphod-concur/etcd,whuwxl/etcd,ajityagaty/etcd,bdarnell/etcd,linuxcer/etcd,luxas/etcd,jonboulle/etcd,ajityagaty/etcd,mickep76/etcd,sofuture/etcd,westhood/etcd,sinsharat/etcd,YuleiXiao/etcd,hongchaodeng/etcd,plasticbox/etcd,bluebreezecf/etcd,insequent/etcd,rnaveiras/etcd,akauppi/etcd,jlamillan/etcd,sebrandon1/etcd,etcd-io/etcd,glycerine/etcd